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274                                      ALUMINIUM

The silica, often contaminated with alumina, is filtered off, dried, ignited
in a platinum crucible and weighed ; it is then evaporated on a water-
bath with a few drops of sulphuric acid and a few c.c. of hydrofluoric acid,
heated to redness and weighed. The loss in weight gives the silica : Si02 x

0-4693 = Si.

5. Determination of the Sulphur, Arsenic and Phosphorus.10

grams of the sample are introduced into a flask fitted with a tapped funnel
and a gas-delivery tube connected with absorption bulbs containing bromine
water, and very dilute hydrochloric acid slowly run in through the funnel
until the metal is completely attacked. The sulphur, phosphorus and
arsenic are oxidised by and retained by the bromine water, one-half of which
is used for the determination of the sulphuric acid by precipitation with
barium chloride. The second half is freed from the excess of bromine
and the arsenic precipitated by means of hydrogen sulphide and deter-
mined as usual. The filtrate from the arsenic precipitate is freed from
excess of hydrogen sulphide and the phosphoric acid then precipitated
with ammonium molybdate (see Determination of the Phosphorus in


6. Determination of the Sodium.35 grams of the sample are
heated gently with nitric acid (D 1-15), the solution evaporated in a porcelain
dish, the residue dried and heated for a long time on a sand-bath, but without
melting the sodium nitrate formed. When cold the residue is taken up in
boiling water, the solution filtered and the filter washed with boiling water,
the filtrate being evaporated to dryness repeatedly with hydrochloric acid
to expel the nitric acid and the residue heated to about 300, allowed to
cool, dissolved in water and the chlorine estimated ; the corresponding
amount of sodium is then calculated.

Lunge2 observes that a little sodium aluminate is formed under these
conditions and advises the treatment of the aqueous solution with ammo-
nium carbonate to precipitate the aluminium and the determination of
the sodium as sulphate in the filtrate.

7. Determination of the Aluminium.0-6  gram  of  the  sample,
reduced to fine fragments, is treated in a flask covered with a small funnel,
with hydrochloric acid diluted with an equal volume of water.    When
the action is complete, the solution is evaporated in a platinum dish on a
water-bath, this evaporation with dilute hydrochloric acid being repeated
several times and the residue finally heated in an oven at 135 to render
the silica insoluble.   The latter is treated with hot water acidified with
hydrochloric acid and filtered into a 250 c.c. beaker, in which it is subjected
to a current of hydrogen sulphide.   The precipitate is filtered off and both
vessel and filter washed with hot water containing hydrogen sulphide, the
filtrate being collected in a 300 c.c. flask, boiled to expel hydrogen sulphide,
treated with a few drops of cone, nitric acid, heated again to oxidise the
iron, cooled and made up to volume.    100 c.c.  of the liquid (= 0-2
gram of metal) are treated in a platinum dish or, fairing that, a porcelain
one, with excess of ammonium chloride and sufficient ammonia to give a

1  Moissan :   Comptes rendus, 1895.

2  Lunge : Technical Methods of Chemical Analysis (London, 1911), Vol. II, p. 348.he washing is carried out first with hydrochloric acid (D 1-2)